Recently, because of the development of integration, it is now practical to use large scale integration (“LSI”) and very large scale integration (“VLSI”) in the fabrication of semiconductor integrated circuits. Furthermore, the minimum pattern of integrated circuit is now at a sub-micron scale, which tends to be a more fine scale one.
In order to form a micro pattern, it is essential to use a photolithography technique in which a photoresist layer is formed on a substrate on which a thin film will be formed, a selective exposure is performed to form a latent image of a desired pattern, a photoresist pattern is made by development, the thin film is etched using the photoresist pattern as a mask, and the desired pattern of the thin film is obtained by removing the photoresist pattern.
Although an ultraviolet ray such as a G-ray (wavelength 436 nm) or an I-ray (wavelength 365 nm) is typically used as the exposure light source in the photolithography technique, a far ultraviolet ray (wavelength 248 nm) with shorter wavelength is used for finer patterns.
Now, a conventional photolithography process using a far ultraviolet ray to form a metal layer pattern is described.
First, an interlayer insulating layer is formed on a structure of semiconductor substrate, i.e., a semiconductor substrate on which a device unit is formed, or a metal line layer, and a metal layer is formed on the interlayer insulating layer.
Next, a metal pattern to form a semiconductor device circuit is formed by the following process: 250 Å of an anti-reflection layer is formed on the metal layer; 50 Å of a protective oxide layer is formed on the anti-reflection layer; a photoresist layer for a far ultraviolet ray is applied, exposed, and developed to form a photoresist pattern; the exposed metal layer is etched using the photoresist pattern as a mask and the far ultraviolet ray as a light source.
At this time, the protective oxide layer is selectively formed. In FIG. 1, an example that an anti-reflection layer 2 is formed on a metal layer 1, on which a photoresist layer 3 is applied using the above conventional method, is shown. In this example, it is not unusual that the photoresist pattern is lifted because the contact layer between the photoresist layer 3 and anti-reflection layer 2 is smooth, which may result in a defective metal pattern.